Why are metal ores dredged from coastal lagoons rather than being extracted from the mother lode?

I want to arrange a situation where the easiest /cheapest way of obtaining metal ores is by dredging them out from the bottom of a series of coastal lagoons. The basic problem being that they are likely to have got there by being washed down stream in rivers, in which case it might be much easier to mine the mother lode on land further up stream.

This situation is based on an Earth like planet (basically the Earth with latitude to move all surface features around an a little wiggle room +/- 10% on average temperature, sunlight and gravity etc). The technology available is as per 1800.

It would be particularly useful if multiple ores could be extracted at the same time. The specific ores in question can be any major ores in use in the 18th century. The crux of the question is how to arrange for lagoon dredging to be the best source.

  • $\begingroup$ It seems like any collection of factors that make one option preferable to the other will be an valid answer to this question. Religious beliefs, political impediments, any number of challenges with site access, extraction, or transportation, or literally anything else that has ever influenced a decision is just as possible a reason. $\endgroup$
    – sphennings
    Commented Jul 8 at 16:53
  • $\begingroup$ What examples of such extraction method being applied in the 19th century has your dilligent research found? Why did they do it? Have you found any examples of such an extraction method using 21st century technology? If not, why isn't it used? $\endgroup$
    – AlexP
    Commented Jul 8 at 17:13
  • $\begingroup$ Please note that before the advent of steam engines, dredging was not something that could really be done on a large scale. It would have been very much easier for them to drain the lagoon(s) and then just dig up the dirt containing metal ores. $\endgroup$
    – AlexP
    Commented Jul 8 at 17:36
  • $\begingroup$ @sphennings True, but some examples would be more contrived than others. Because the local priest/lord says no is a bit lame. Perhaps the motherlode is inaccessible for some geological reason, but the sediments aren't is more like it. $\endgroup$
    – Slarty
    Commented Jul 8 at 17:49
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    $\begingroup$ Different level of technology but iirc roughly since the 1980s or so they've got diamonds off the seafloor of Namibia despite already mining where they originated simply because they've already accumulated over 100s of years and so there's no reason not to simply take them from both places $\endgroup$
    – OT-64 SKOT
    Commented Jul 11 at 6:50

8 Answers 8


The same reason we extract many ores from such deposits, they are natural accumulations.

Either from biological action (banded iron) or sedimentation accumulation (laterites and placer deposits) ore from low density sources are concentrated in sedimentary rock. These sources are often richer than the source material by orders of magnitude since source rock can be spread over huge areas. this works for metals, metal oxides, sulfides and silicates.

Specifically your deposits would either be a paleo-beach placer or a paleo-alluvial placer. this will give it a high concentration in a lagoon environment.

There is no motherlode; your deposit is a concentration of something normally present is very small quantities uniformly in the parent rock that get concentrated by erosion.

  • 1
    $\begingroup$ Malaysia tin mines were generally of this type, with dredging a common way of getting the ore. $\endgroup$
    – Jon Custer
    Commented Jul 9 at 17:31
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    $\begingroup$ Or maybe there was once a motherlode, but it all collapsed and was washed out into the river delta. $\endgroup$ Commented Jul 9 at 19:05

Gold panning is a traditional process that was well-known in the 1600's. There are variants of the round pan. The Spanish batea was traditionally made from a piece of wood. The design was used where there was less water available then for regular gold panning. The Japanese yuri-ita was a similar device: a rectangular wooden plate, open at one edge. It is a small leap from that to the shaking table used for isolating ores by density. These were used in mines towards the end of the 1800s to purify low grade crushed ores. I have seen them used in tin mines. They could also be used to sort out alluvial deposits with heavy metal components. If your people have a lot of dirt to sort through to get at a small amount of heavy metal and particularly if they have a limited clean water supply, it would be reasonable to bring forward the invention of the shaking table by a hundred years. Indeed - they may have been used earlier than that, but most of the history now seems to belong now to the people who filed the first patents.

Why would someone sort through a lagoon? Why pan for gold? The mother lode source may be unknown. It may be inaccessible. It may be too much work to break up the quartz to get at the little grains of gold. The mountains may be in someone else's territory.

Could you sort for multiple metals or ores? It can be done. However, your alluvial deposits have probably already been separated by a similar process. If you know a bend in the river where particles of gold collect, it is likely that other materials may have been swept elsewhere.

A plausible history might start with people panning for gold on a river in the known 'good spots'. A larger scale enterprise may then dam the river further down, and try to extract gold in smaller concentrations. As they improve their purification process, they may find other identifiable materials leaving the shaking table at different points that they can also use.

  • $\begingroup$ Gold is almost always found in native state. I have never heard of a gold ore being exploited. $\endgroup$
    – AlexP
    Commented Jul 8 at 17:16
  • $\begingroup$ Gold is extracted as a metal. Tin is extracted as an ore. The principle is much the same. $\endgroup$ Commented Jul 8 at 17:20
  • $\begingroup$ The principle is much the same except it is not. Gold is a metal. It is heavy. It can easily be separated by primitive flotation techniques. Iron or tin or whatever ores are oxides, or, as they called them in pre-modern chemistry, earths. (Hence the name of the "rare earth metals", that is, metals of which the oxides were not common.) They are basically dirt, very hard to separate mechanically from other kinds of dirt. $\endgroup$
    – AlexP
    Commented Jul 8 at 17:24
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    $\begingroup$ @AlexP placer deposits are common for almost all metals, including oxides and sulfides. "dirt" as you call it has diffrent densities calsing it ot be easily seperated by erosional processes, please learn some geology. $\endgroup$
    – John
    Commented Jul 8 at 17:57
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    $\begingroup$ @AlexP gold ore mining is a real thing $\endgroup$
    – Seggan
    Commented Jul 10 at 2:47


This is exactly how bog iron works in real life. Bog iron was the primary source of iron ore throughout most of Europe's history. Mineral iron refining did not become common until the industrial era when the tools needed to break up the harder stones that were typical of hematite and manganite became better, and the demand for higher volumes and newer sources of iron made it necessary.

With bog iron, you have an upstream source of rocks which may have very low iron concentrations, sometimes even less than 10%, but over time, rain breaks this rock down where its sediment flows into a bog. There, certain kinds of bacteria consume the iron and clusters together in colonies. When they die they leave behind concentrated iron resulting in new rocks that are about 50-70% iron.

This accumulation of iron by bacteria in bogs means that you can burn several times less fuel to smelt the same amount of iron than if you were to try mining it at its source. In most cases of large scale bog iron mining, the preferred method was to drain the body of water then to mine the dried up bog, but in some cases, there is no good way to drain a bog, or the local demand of iron is not high enough to support such a major terraforming project so dredging is used instead.

Sedimentary Alloying

In addition to bioaccumulation, bogs also create the conditions for a specific sedimentary layer upstream to accumulate and mix with sediments from another upstream source creating ores that form a wide range of natural alloys. Before the advent of modern chemistry, people knew that some ores were naturally better than others but not why. Nordic and Spartan ores were naturally rich in manganese, India has ores rich in chromium and vanadium, etc. But most of these ores exist because of converging sources of sediment flowing into a still body of water where they are allowed them to mix and accumulate. These ores tend to not be as pure or cheap to refine as bioaccumulated ores, but the exact mixture could be more desirable than either upstream source. So even if you have an upstream "motherlode" of hematite that you know about and can mine, dredging a certain bog where it's sediment mixes with other desirable elements could yield a clay containing a superior mixture of elements.

Because this is an Alien World...

On Earth, Iron, Mercury, Chromium, and Arsenic are the 4 main elements that bioaccumulate, and they tend to bioaccumulate more or less independently of one another, but if you are following alien biology, bioaccumulation could do all sorts of things. You could have a microorganism that naturally bioaccumulates a useful mixture of iron, manganese, and chromium. Or you could have lots of specialized bioaccumulators that like to cluster into distinct colonies such that a single bog might have several useful isolated elements. So, when you dredge the bog you get ingots of gold, silver, copper, iron, etc. that you can simply sort out from one another.

  • $\begingroup$ "Bog iron was the primary source of iron ore throughout most of Europe's history": Europe is not Japan. I do not know of any major European historical civlization which used bog iron as its primary source. Or even as its secondary source. Or tertiary source. Not the ancient Grecians. Not the ancient Romans. Not medieval France or Germany or Spain. Can you please give an example of one major medieval European kingdom, republic, whatever, which got most of its iron from bogs? $\endgroup$
    – AlexP
    Commented Jul 8 at 17:21
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    $\begingroup$ @AlexP most of europe for the 1st century BCE, but esspecially the Scandinavia region. bog iron is easier to smelt than most other sources. Also Russia until the 16th century. $\endgroup$
    – John
    Commented Jul 8 at 17:52
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    $\begingroup$ @AlexP Looks like the Romans and Greeks also got most of thier iron from bog iron (AKA: Limonite). The 8 major iron production sites in ancient Greece and Rome that I've been able to find information on had local Limonite, but they did not all have local Hematite or Magnetite. And the 2 regions I could find slag analysis information on were both Limonite refining sites. I actually could not find any evidence of Greece or Rome using Hematite or Magnetite. Even though you read about Greek and Roman "iron mines", the physical evidence suggests that at least most of those were bog iron mines. $\endgroup$
    – Nosajimiki
    Commented Jul 8 at 20:04
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    $\begingroup$ @AlexP IYou will find "barbarians" were perfectly capable of smelting iron. here are some sources if oyu would like to learn some history. onlinelibrary.wiley.com/doi/10.1111/1475-4754.00016 and researchgate.net/publication/… $\endgroup$
    – John
    Commented Jul 8 at 23:20
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    $\begingroup$ @AlexP Several of the mine sites used by the Romans have high water tables today, but I did not find any references to iron mines in notably dry regions. Also, Limonite is softer than hematite/magnetite; so, the argument that it is easier to mine holds true even in an old dried up bog. That said, I've come across a lot of descriptions of ancient iron mines over the years that talk heavily on the technologies various cultures used to drain them, and the watery runoff that comes from them after they are abandoned. This further implies that they tended to be in wet areas. $\endgroup$
    – Nosajimiki
    Commented Jul 9 at 16:29

The original deposit can be extremely deep and sparse. Groundwater takes deep detours and can carry minerals from kilometers deep.

If we really stretch, it can be that the mineral is water soluble in its basic form, but not after oxidation. When the water stream surfaces from a well, the mineral oxidizes in contact with outside air, and quickly deposits in the first still body of water.


Same reason it's sometimes done on earth: someone else controls the location of the motherload.

Under those conditions, you take what you can, however you can.


Simple accessibilty is enough to explain this. Being "on land" doesn't make the motherlode more accessible; if the motherlode is actually far beneath the land, then it can be exceedingly difficult and dangerous to reach. On the other hand, water can pass pretty deep into the earth before making its way to the sea. On the way, it can bring all sorts of minerals and materials with it - things that would be very hard to access directly.

In the late 1700s and early 1800s, the deepest mines were 200m deep, maybe 250m. There are places in the world where groundwater and aquifers penetrate as deep as 3000m. It's quite possible that the motherlode is an order of magnitude deeper than these people can safely mine, but the groundwater flows through those rocks and transports quantities of the material with it as it flows and reemerges elsewhere in countless tiny springs and streams. Those streams exhibit small, dispersed quantities of the ore, but they all flow downhill. If the local terrain is such that these streams flow towards the same area of coastline, then this will collect that ore into a smaller and smaller area. Eventually, the rivers all empty into coastal lagoons, where the calm waters allow them to deposit all they've carried, and the deep layer of soft silt and mud will trap things like fragments of stone and ore.

This makes the lagoon mud the perfect place to look for ore:

  • it contains material from far deeper underground than these people are capable of mining.
  • it contains a far greater concentration of the ore than any of the individual streams and rivers that brought the ore here.
  • a lagoon is safe and sheltered, making it easy to put large barges and dredging vessels on.
  • the silt and mud dredged out of the lagoon is soft and easy to process and extract ore from, possibly as simple as just passing it through a sieve.

Depending on their level of scientific understanding, tt may be that the people are aware of all this, or it might be that they haven't yet determined where the ore comes from. It might be that they simply noticed long ago that this mud sparkles and contains grit and stone particles, and in olden times the locals might have extracted small quantities of it by hand using sieving and panning techniques, and smelting it in simple pit furnaces. In modern times, the process is industrialised. They wouldn't necessarily even have a need to seek the motherlode unless they started extracting the ore faster than it was being deposited, to the point that they exhausted the lagoon deposits.

In fact, it's possible that there isn't a single "motherlode", but instead a dispersed series of low-concentration veins and deposits. This process of transport and accumulation by water could remove ore from the rock and soil of a very large area and bring it all into a dew small lagoons, which could make these lagoons the greatest concentration of usable ore there is, offering more ore per tonne of material than the rock that the ore came from originally.


The motherlode is in a very unstable area. Even relatively small excavations cause the surrounding dirt/sand to collapse, so for every 10 ft (for example) you dig towards the lode, another 8-10 ft fills in your hole. And normal methods for holding back collapses don't have anything to anchor to, so they end up shifting in dangerous ways or simply collapsing, too.

Since you are looking at technology around 1800, there's little in the way of large industrial machines, so most of the digging is by hand or rudimentary machines, so it simply takes too much time and effort to dig, at least compared to the easier to access ore under water. Mining machines exist, but are rare and expensive. They won't become common until the later half of the century.


Yes, you can throw more people at it, but that gets expensive fast, especially since you don't actually know where the lode is or even how large it is, and you aren't getting anything out of the excavation. You've gotten investors before, but they quit sending good money after bad.

You've also lost a considerable amount of workers in accidents due to the unstable landscape, so people are wary of the job site and you have fewer and fewer people at the mine every week, so you can't even hire more people if you wanted to.

Dredging has been around since around 1575, so the technology is fairly mature for your needs. And it's pretty quick, at least compared to the alternative.


You've sent divers to check out how much ore is available. They don't have SCUBA gear, so they can't stay down super long, but they tell you that the ore is spread far and wide. You've also done core samples at random places, which shows that the bottom of the river delta has a very thick layer of ore, so you don't need to go through the expense of digging out the motherlode anytime soon. Your engineers assure you that it'll be decades before you come even close to running out of the easy to access ore.

Alternative options

Other issues that can come up is that the motherlode is in another jurisdiction than the mining community. Or they simply don't have access to it.

Maybe a competing company owns the land, but doesn't know about the lode, and the company doesn't want competition by bringing attention to it.

Maybe it's in a community that forbids commercial mining.

Maybe it's owned by some rich people that want to keep the view from their home pristine, rather than have a large strip mine tear down the trees.

Maybe there's a local native community living on it. Either they are hostile and no one comes beck from trying to negotiate, or they are extremely beneficial to the community and no one wants to risk their generosity. Maybe it's on land sacred to the natives and they guard it ferociously regardless if they are normally amiable.

Maybe there's already significant housing or a thriving industrial section on it and no one wants to tear it up to get to the lode.

Maybe there's public resistance to tearing down a well known landmark.

Maybe if the motherlode area is damaged, the river dries up or the river alters course.

I'm sure there are many other alternatives that I haven't mentioned.

Multiple ores

In order to pull out multiple types of ore, you need chemistry. Most of the stuff pulled out of the ground is mixed with something else. Ancient Egyptians were able to get gold to 95% pure, but whether they were able to use the silver, copper, and other impurities they pulled out of it, I don't know.

"The alloys used to produce the studied jewellery[sic] range from high purity ones, with a gold content that can reach 95%..."

Even more recently, slag from coal mines are now being processed to get minerals from them that was previously ignored or unable to be accessed, but this wasn't done until fairly recently, but I can't seem to find any dates in any articles I've found about the process.



Rapid Shoreline Erosion.

If your world experiences more rapid climate change, with the ocean levels changing quickly, heavy rainfall and cold winters in some centuries, and then dry, hot climate on others, then the shores of the continents would erode and constantly, faster than they can renew. The sea would essentially strip away rock until the less brittle, and more maleable motherlodes of metals are exposed.

Take for example, Rio Tinto in Spain. Its shores are iron rich sandstone, continuously being eroded by the river. It washes out so much iron to dye the river orange. If the same kind of rock was exposed not to the relatively lazy river, but savage blasts from the Atlantic, the sandstone would be ground away, leaving a lagoon of rust. The only reason why we do not see this happen a lot IRL is that seaside erosion on Earth is more or less evenly matched by sediment deposition, so for every mile of metal rich rock exposed, another mile is buried under the sand. But one can easily envision a planet where erosion is worse for many reasons, while the eroded rock particles not plastered back over the shores, but sink to the bottom of the ocean, not to return for millions upon millions of years.

In such a world, you would not need to dig for metal ores, the ocean itself would be continuously strip-mining the land, all you need is a big crew with sieves and snorkels to get it.


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